This project will evaluate the effectiveness of a high-penetration, high- volume needle exchange program 1) in reaching out-of-treatment drug injectors, 2) in reducing HIV risk behavior, 3) in averting new HIV infections, and 4) in referring drug injectors to drug treatment and HIV testing and counseling. The proposed evaluation of the high volume exchange will take advantage of a natural experiment in that we will gather baseline data both before and after implementation of the high volume exchange. The evaluation attempts to redress the shortcomings of previous evaluations of needle exchange, specifically, the difficulty of correcting for self-selection to the exchange in estimating the impact of needle exchange on HIV risk behavior. In addition to assessing penetration of the exchange of a population of out-of-treatment drug injectors, we will investigate the extent to which attendance at the exchange mediates risk reduction and decreased seroincidence. We will conduct both serial cross-sectional surveys, and a prospective cohort study that will enable us to compare the behavior of attenders and nonattenders, holding constant behavior prior to initiation of the exchange. The data gathered in interviews with community drug injectors will also be used to estimate mathematical models of the number of HIV infections averted by the exchange, and the cost-effectiveness of the exchange in terms of its cost per averted infection. Finally, we will evaluate the success of the needle exchange program in referring exchange clients to drug treatment and HIV testing and counseling. The approach to needle exchange is based on the principles of penetration (reach) and volume. Volume will be attained by daily exchanges, which will greatly increase opportunities for exchanging. Penetration will be achieved by making the exchange convenient to attend and by having outreach workers make referrals to the exchange. To maximize convenience, the route of a mobile van housing the exchange will be informed by detailed information the outreach team has about the location of pockets of injection drug use in the community. This project is a collaboration of the Santa Clara County AIDS Program, the HIV Prevention Center of Haight Ashbury Free Clinics, and the Center for AIDS Prevention Studies and Institute of Health Policy Studies at the University of California, San Francisco.